[exim] verifying errors_to

When I implement a simple mailing list by an example as in chap 51.2
of the docs, why does the errors_to address have to be verifiable by a
router that comes *before* the list router?

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## subscription configuration (requires account):
## https://lists.exim.org/mailman3/postorius/lists/exim-users.lists.exim.org/
## unsubscribe (doesn't require an account):
## exim-users-unsubscribe@???
## Exim details at http://www.exim.org/
## Please use the Wiki with this list - http://wiki.exim.org/